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In this article we consider the complete set of synchronized and phase-locked states available to pairs of hydrodynamically coupled colloidal rotors, consisting of spherical beads driven about circular paths in the same, and in opposing senses. Oscillators such as these have previously been used as coarse grained, minimal models of beating cilia. Two mechanisms are known to be important in establishing synchrony. The first involves perturbation of the driving force, and the second involves deformation of the rotor trajectory. We demonstrate that these mechanisms are of similar strength, in the regime of interest, and interact to determine observed behavior. Combining analysis and simulation with experiments performed using holographic optical tweezers, we show how varying the amplitude of the driving force perturbation leads to a transition from synchronized to phase-locked states. Analogies with biological systems are discussed, as are implications for the design of biomimetic devices.

A system that allows the collection of the low loss spectrum and the core loss spectrum, covering different energy regions, at each pixel in a spectrum image is described. It makes use of a fast electrostatic shutter with control signals provided by the spectrum imaging software and synchronisation provided by the CCD camera controller. The system also allows simultaneous collection of the X-ray spectrum and the signals from the imaging detectors while allowing the use of the existing features of the spectrum imaging software including drift correction and sub-pixel scanning. The system allows acquisition of high-quality spectra from both the core and the low loss regions, allowing full processing of the EELS data. Examples are given to show the benefits, including deconvolution, absolute thickness mapping and determination of numbers of atoms per unit area and per unit volume. Possible further developments are considered.

Patients who have had one oral cancer are at increased risk of developing a further malignant tumour, the detection of which is made difficult (and is often delayed) by the innocuous appearance of the early oral lesion. A technique that could reliably detect early cancers would be useful to both oral and dental health specialists. We describe a pilot study in which we used a compact spectroscopic instrument designed to excite and measure fluorescence in the oral cavity. The data were processed using principal components analysis, and the results suggest that the technique might be valuable for detecting early oral cancers. Further work should be performed to investigate some unusual characteristics observed within our data to ascertain if these are significant, simply due to errors made due data collection, or are due to other lifestyle factors. Such work could also verify that the data are due to detection of ALA metabolite in cancer and not some other systemic effect.

Background: In recent years, 5-aminolaevulinic acid (ALA) has become an increasingly popular photosensitizing drug for use in both photodynamic therapy (PDT) and photodetection (PD) of cancers. ALA metabolizes within tissue to form the photosensitizer protoporphyrin IX (PpIX). Like most photosensitizers, PpIX is fluorescent, and this fluorescence progressively decreases during PDT. This phenomenon is referred to as photobleaching., Aim: Our aim in carrying out this experiment was twofold: firstly, to compare the relative capacity of red and blue light to cause photobleaching; and secondly, to compare the capacity of a fixed light dose to cause photobleaching, when delivered at different intensities., Method: In this paper, we describe the implementation of a compact fluorescence spectrometer in monitoring the photobleaching of ALA-induced PpIX in vivo on the skin of healthy volunteers., Results: We have been able to show that blue light causes more rapid photobleaching than red light, and that under illumination with red or blue light, delivery of a fixed light dose at a lower intensity results in more photobleaching., Conclusion: Comparison of the photobleaching rates suggests that a blue light intensity of 5 mW/cm(2) gives the same rate of photobleaching as the typical red light PDT intensity of 100 mW/cm(2). Further investigation of the correlation between PpIX photobleaching and PDT effect would be beneficial in interpreting the clinical significance of our findings.

We explain that, unlike the spin angular momentum of a light beam which is always intrinsic, the orbital angular momentum may be either extrinsic or intrinsic. Numerical calculations of both spin and orbital angular momentum are confirmed by means of experiments with particles trapped off axis in optical tweezers, where the size of the particle means it interacts with only a fraction of the beam profile. Orbital angular momentum is intrinsic only when the interaction with matter is about an axis where there is no net transverse momentum.

A novel mechanism is proposed for the generation of self-similar structure over a limited range of length scales. Our mechanism, which we call the monitor-outside-a-monitor effect, comprises repeated magnification and addition of small-scale structure. We invoke this mechanism to explain recent observations of fractal structure in the eigenmodes of unstable optical resonators [G. P. Karman et al., Nature (London) 402, 138 (1999)].

A large neutrino asymmetry could have significant observable consequences for nucleosynthesis and the cosmic microwave background. If the baryon asymmetry originates via the Affleck-Dine mechanism along a d = 4 flat direction of the scalar potential in the minimal supersymmetric standard model and if the lepton asymmetry originates via Affleck-Dine leptogenesis along a d = 6 direction, corresponding to the lowest dimension directions conserving R parity, then the ratio n(L)/n(B) is naturally in the range 10(8)-10(9). As a result, a potentially observable neutrino asymmetry is correlated with a baryon asymmetry of the order of 10(-10).

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.

The worldwide endeavor to build long baseline laser interferometers to detect and study gravitational radiation is well under way. In the German-British GEO600 project, it is proposed to pass the sidebands induced on the light by an electro-optic phase modulator through a Fabry-Perot optical cavity used in transmission, called a mode cleaner. This can be achieved when the phase modulation frequency is matched to the first longitudinal-mode frequency of the mode cleaner cavity so that both carrier and sidebands are transmitted. The primary function of the mode cleaner is to reduce the geometry fluctuations associated with the light, and thus any such noise induced by the modulation process is also suppressed. We present the results of an experiment that investigates the feasibility of passing modulation sidebands through an optical cavity and the factors limiting its success. In particular, we show that it is possible to avoid introducing excess noise associated with the transmitted sidebands, provided that certain experimental criteria are satisfied. The research was carried out on a prototype mode cleaner cavity built and tested at Glasgow University but which is similar to the equivalent apparatus planned for GEO600.